Drayton Digistat 1?

For the 1st time since I moved in to my house I had to change the batteries in my Drayton Digistat 1 (the old version I believe as the sticker inside says the warranty ran out Mar '05).

Whilst the battery indicator has now disappeared and the thermostat seems to be working OK i.e. it shows the current room temperature, it no longer seems to call for heat despite the flame showing on the thermostat. Everything worked fine before I had to remove from wall to change the battery (the slide down battery compartment was too stiff to withdraw whilst in situ).

My boiler is a back boiler and this is working fine i.e. heating water and operatuing as a fire so I don't believe that there are any problems there.

If anyone can provide me with some advice that would be much appreciated, failing that I shall just have to purchase a new thermostat and hope that the fault lay in the old unit and nowhere else.

Thanking you in anticipation,

Adam